Oxygen may be toxic to spermatozoa probably via lipid peroxidation and thus membrane disruption. Hypotaurine, which occurs in millimolar concentration in mammalian seminal fluid, has been shown to (1) protect spermatozoa from lipid peroxidation and (2) maintain sperm motility. We propose to study the factors which determine the steady state concentration of hypotaurine in seminal fluid. These include two enzymes involved in the biosynthetic pathway (cysteine dioxygenase and cysteine sulfinate decarboxylase) and two enzymes catabolizing hypotaurine (hypotaurine transaminase and hypotaurine "oxidase"). Castration has been observed to result in the loss of hypotaurine in animal seminal fluid and replacement testosterone returns the hypotaurine concentration back to normal. Thus, we propose to examine which biosynthetic enzyme activity declines with castration and returns to normal after replacement testosterone. We also propose to synthesize and test a "suicide" type inhibitor targeted to specifically destroy cysteine sulfinate decarboxylase, the enzyme responsible for hypotaurine biosynthesis. Insights, leading from these studies, are applicable in understanding problems with human fertility as well as lead the way to developing potential male contraceptive agents.